Salinity-Induced Inhibition of Lettuce Seed Germination in Association with Altered Catalase and Endo-β-Mannanase Activities

Salinity is a major constraint on lettuce (Lactuca sativa L.) production and is known to inhibit seed germination. However, the physiological and biochemical processes underlying this sensitivity remain unclear. Therefore, this study aimed to investigate how salinity affects seed germination in two lettuce cultivars, ‘Susan’ (a highly salt-sensitive cultivar) and ‘Yafa’ (a low salt-sensitive cultivar), with particular emphasis on the roles of catalase and endo-β-mannanase enzyme activities. Seeds were subjected to both low salinity (0, 0.1, 0.2, 0.3, 0.5, 1, 3, and 5 mM NaCl) and high salinity (0, 10, 20, 40, 80, 160, and 320 mM NaCl) under standard germination conditions to evaluate germination percentage, mean germination time, and enzyme activity. Seedling emergence was also assessed in different growing media, including perlite, sand, peatmoss, and cocopeat. The results showed that salinity significantly reduced germination percentage and seedling length and increased mean germination time, with inhibition occurring at ≥0.1 mM NaCl in ‘Susan’ and ≥40 mM NaCl in ‘Yafa’; both cultivars failed to germinate at 320 mM. The ’Yafa’ had a high seedling emergence in all growing media, but ’Susan‘ seeds only emerged in perlite, which had the lowest salinity. Catalase activity increased markedly under salt stress, particularly in ‘Susan,’ indicating elevated oxidative burden, while endo-β-mannanase activity declined with increasing salinity, especially in the highly salt-sensitive cultivar of ‘Susan’. Correlation analysis showed that germination percentage had a significant and positive correlation with endo-β-mannanase activity and had a significant and negative correlation with catalase activity across salinity levels. In conclusion, salinity-induced inhibition of lettuce seed germination appears to be associated with changes in antioxidant enzyme activity and reduced endosperm weakening capacity, as reflected by altered catalase and endo-β-mannanase activities, thereby contributing to cultivar-dependent differences in salt sensitivity.